The aim of this symposium is to bring together scientists from diverse fields such as immunology, endocrinology, pharmacology and cell biology, but with common interest in the signalling machinery mediated by cell surface receptors. The progress that has been made in each one of these disciplines will be evaluated and compared with that of the other fields. Different concepts and technical strategies will be discussed in an attempt to achieve a mutual fertilization and to create a general scheme which will enhance the progress of future studies. We feel that the knowledge accumulated in each one of the disciplines which explore receptor activation, must be ultimately shared by the other fields. It is clearer now than at any time before, that the principles of receptor activation are similar even in cells with distinctive functions. Therefore, an integrated symposium on "Receptor Activation by Antigens, Cytokines, Hormones and Growth Factors" should have a direct and immediate impact on all the interested participants. To allow active involvement of registered participants and encourage free discussion among participants from different disciplines the following integrated topics will be discussed in three special sequential sessions (Thursday afternoon): l. Receptors' structure and function. 2. Ion fluxes and second messengers. 3. Signal transduction pathways.